Single-cycle multi-terahertz spectroscopy: observing the build-up of phonon–plasmon coupling in photoexcited InP

Abstract

Exploiting ultrabroadband terahertz technology, we study the femtosecond transition of a longitudinal optical phonon resonance to a coupled phonon–plasmon system. After 10 fs photoexcitation of intrinsic InP, the build-up of coherent beats of the emerging hybrid modes is directly monitored in the time domain with subcycle resolution. Mutual repulsion and redistribution of the oscillator strength of the interacting phonons and plasmons are seen to emerge on a delayed femtosecond time scale. Both branches of the mixed modes are monitored for various excitation densities N. We observe a pronounced anticrossing of the coupled modes as a function of N. The characteristic formation time for phonon–plasmon coupling exhibits a density dependence. The time is approximately set by one oscillation cycle of the upper branch of the mixed modes.